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The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source. Examples of active laser media include: *Certain crystals, typically doped with rare-earth ions (e.g. neodymium, ytterbium, or erbium) or transition metal ions (titanium or chromium); most often yttrium aluminium garnet (Y3Al5O12), yttrium orthovanadate (YVO4), or sapphire (Al2O3);〔Hecht, Jeff. ''The Laser Guidebook: Second Edition.'' McGraw-Hill, 1992. (Chapter 22)〕 *Glasses, e.g. silicate or phosphate glasses, doped with laser-active ions;〔Hecht, Chapter 22〕 *Gases, e.g. mixtures of helium and neon (HeNe), nitrogen, argon, carbon monoxide, carbon dioxide, or metal vapors;〔Hecht, Chapters 7-15〕 *Semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN).〔Hecht, Chapters 18-21〕 * Liquids, in the form of dye solutions as used in dye lasers.〔F. J. Duarte and L. W. Hillman (Eds.), ''Dye Laser Principles'' (Academic, New York, 1990).〕〔F. P. Schäfer (Ed.), ''Dye Lasers'', 2nd Edition (Springer-Verlag, Berlin, 1990).〕 In order to lase, the active gain medium must be in a nonthermal energy distribution known as a population inversion. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.〔(Encyclopedia of laser physics and technology )〕 ==Example of a model of gain medium== A universal model valid for all laser types does not exist.〔 〕 The simplest model includes two systems of sub-levels: upper and lower. Within each sub-level system, the fast transitions ensure that thermal equilibrium is reached quickly, leading to the Maxwell–Boltzmann statistics of excitations among sub-levels in each system ''(fig.1)''. The upper level is assumed to be metastable. Also, gain and refractive index are assumed independent of a particular way of excitation. For good performance of the gain medium, the separation between sub-levels should be larger than working temperature; then, at pump frequency , the absorption dominates. In the case of amplification of optical signals, the lasing frequency is called ''signal frequency.'' However, the same term is used even in the laser oscillators, when amplified radiation is used to transfer energy rather than information. The model below seems to work well for most optically-pumped solid-state lasers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Active laser medium」の詳細全文を読む スポンサード リンク
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